The present invention relates to prototyping processes and, more particularly, to a composite tooling process for curing materials at elevated temperatures.
The production of molded articles from resinous fiber materials, especially large and complex articles such as, for example, airplane fuselages, has been subject to cost, complexity, and consistency problems. Especially in a prototyping stage where few articles are produced from the mold, costs, lead-time, and accuracy of the articles produced from the mold are extremely important considerations.
An example of a current process used to produce prototyping cure tools begins by obtaining large xe2x80x9cblocksxe2x80x9d of 10-15 lb density urethane foam. The foam buns are attached to a base and glued together to form a generic shape. The glue used is, for example, a room temperature laminating system. The foam is then machined to the desired shape using a numerically-controlled (N/C) machine to thereby obtain the desired mold line shape of the pattern or object to be molded. The foam shape is then sealed using a room temperature resin system before the resin is sanded and coated with an automotive type paint primer. Once the master plug or facility tool is formed, two methods are typically used to fabricate the facing sheet of the bond jig or cure tool. In one method, a room temperature cure/high temperature use (RT/HT) resin is used in a pre-preg (material already impregnated with the (RT/HT) resin), wherein the pre-preg is laid up in material sheets on the facility tool and cured out at room temperature or low temperatures (less than 150xc2x0 F.). A second method for fabricating a facing sheet of the cure tool is to use resin and dry cloth and hand impregnate (apply the resin to the cloth) during the lay-up process. Typically, the cure tool or bond jig is then attached to a carbon epoxy eggcrate structure and removed from the facility tool.
Several limitations and/or problems are associated with such a method for producing cure tools. For example, in the first xe2x80x9chalfxe2x80x9d of the process, the large blocks of the urethane foam usually carry an accompanying manufacturer lead-time of up to approximately 6-8 weeks, with 3-4 weeks lead-time being about average. A further problem is that the foam generally does not machine well. This is evidenced in that measurement variances of the machine molds are often much greater than the acceptable tolerance of within 0.010 inches of the theoretical mold line. The poor machinability of the foam may be due to two different problems therewith. First, the foam structure can be considered as a plurality of foam balls glued together. As the foam structure is machined, the balls are xe2x80x9ctornxe2x80x9d from the glue instead of being cut to form the desired contour. Thus, in actuality, the foam balls are being pulled off the surface of the structure instead of being actually cut, thereby resulting in a rough surface finish following the machining process. A second additional problem stems from the fact that the foam balls pulled from the surface of the structure during the machining process are actually abrasive and contribute to additional machining of the foam structure from a grinding process in the vicinity of the cutter.
After the foam mold is machined, resin is applied to the foam structure to fill and smooth out the surface. The resin is added about the foam structure, sanded smooth, and coated with an automotive-type paint primer before the surface is hand worked to achieve a smooth finish. However, the dimensional accuracy of the prototype tool is typically unknown due to the extensive handwork involved in finishing the facility tool. In addition, the vacuum integrity of these foam tools is generally hard to achieve. If the tooling surface facility tool cannot be subject to a good vacuum without leakage therethrough due to poor consolidation of the tool structure or other factors, the cure tool produced from the facility tool may be unusable. Further, these foam tools are also susceptible to damage. Any loads applied to the tools must be evenly distributed in order to avoid damage to the mold. Thus, there exists a need for a method of making a cure tool wherein extensive handwork is not required to produce an acceptably smooth finish on the cure tool, thereby resulting in a more determinable dimensional accuracy thereof. Further, there exists a need for a method of forming a cure tool wherein the tools or molds used in the process have good vacuum integrity. In addition, there exists a need for a method of forming a cure tool wherein the facility tool and the cure tool are sufficiently strong to withstand damage due to uneven forces applied thereto.
Further limitations in typical prototyping processes are encountered in the fabrication of the facing sheet of the cure tool or bond jig. Typically, using a room temperature cure/high temperature use (RT/HT) material, the time required to lay-up the tool is less with the pre-preg material than with the hand-impregnated material. Further, though the curing of these materials can be performed at room temperature, this usually produces a poor quality tooling that results in poor surface quality and/or poor vacuum integrity. Curing a pre-preg tool at elevated temperatures produces a better quality tooling. However, the same problems of poor surface quality and poor vacuum integrity may still exist, albeit to a lesser extent. The best results for RT/HT tools are typically obtained by curing the tools at temperatures up to 150xc2x0 F. and at an elevated pressure. However, the size of the mold may be a limiting factor in this situation where the size of the temperature/pressure chamber may be limited. In addition, foam facility tooling materials generally have a high coefficient of thermal expansion. Thus, compensation during the machining process of foam molds must be made in order to allow for expansion of the mold when cured or used at high temperatures. Further, since the foam is typically glued up from smaller pieces or buns of the foam to form the mold, the growth of the mold at elevated temperatures is not uniform. In this regard, the bond or glue lines between the foam buns will not grow at the same rate as the foam itself.
Hand impregnation using a RT/HT resin does increase the time necessary to fabricate the lay-up, but the quality of the mold produced increases accordingly. However, the hand impregnation method is also dependent on the vacuum integrity of the foam mold. If the foam is not able to pull a sufficient decreased pressure or vacuum, and maintain that decreased pressure, a poor quality cure tool will result. In addition, the hand impregnation method requires a debulk of the plies at every few layers. This increases the number of required debulks as compared to a pre-preg material. Further, the use of a vacuum bagging material around the edges of the mold can result in damage to the surface coat and necessitate additional time for repairs during the lay-up of the tool. Thus, there exists a need for a method of forming a cure tool or bond jig wherein the tooling produced has good surface quality and vacuum integrity after being cured at room temperature. In addition, there exists a need for a method of fabricating a cure tool or bond jig wherein compensation for expansion of the facility tool at high temperatures is minimal or not required. However, if the facility tool were to expand as a function of temperature, it would be preferable for the mold to expand uniformly. In addition, there exists a need for a method of forming a cure tool wherein the vacuum bagging material used to cover the facing sheet and to seal to the facility tool during the curing process for the facing sheet can be removed therefrom without damage to the facing sheet.
The fabrication of molding tools has been addressed in a number of ways. For example, U.S. Pat. No. 4,073,049 to Lint discloses a method of making a mold for vacuum thermoforming which consists of applying a gel coat to a master pattern, applying a rigidizing mixture of a thermosetting resin and glass to the gel coat, constructing and bonding an eggcrate framework to the cured thermoset resin and glass fiber mixture, filling the spacings of the eggcrate framework with a polymeric rigidizing foam, forming a vacuum plenum on the foam-filled eggcrate, attaching a vacuum plenum thereto, and forming air passageways through the gel coat which communicate with the vacuum plenum. However, the vacuum forming mold plug disclosed by the ""049 patent generally comprises a spray-up mold shell formed of standard thermosettable resins such as a polyester resin and chopped glass fiber. Further, the reinforcing eggcrate framework is preferably formed of wood and then bonded to the mold shell before the internal spacings in the eggcrate structure are filled with a polymeric rigidizing foam. The ""049 patent further discloses that the resulting mold is sufficient for forming plastic materials of 0.250 inches or less in thickness which are required to be heated to 350xc2x0 F. However, the use of a resin/glass fiber spray-up to form the mold shell, the wooden eggcrate framework, and the polymeric foam fill, results in a structure having differing coefficients of thermal expansion between the elements. Thus, the vacuum forming mold plug disclosed by the ""049 patent may possibly become distorted upon heating during the thermoforming cycle and result in questionable vacuum integrity and dimensional accuracy thereof. Further, the use of a central vacuum plenum pulling a vacuum on the article to be molded through a plurality of air passageways extending through to the mold shell may also contribute to a poor surface finish on the molded article as well as possible distortion due to the uneven distribution of the vacuum.
U.S. Pat. No. 4,834,929 to Dehoff et al. discloses a method of making molds by applying a plurality of layers of materials to a pattern. The method includes providing a pattern of the part to be formed, orienting the pattern on a surface plate in a molding dam, applying a release coating to the surfaces, applying a thin resin layer to form the mold surface, applying a layer of fiber reinforced tooling resin to the thin resin layer, applying an epoxy-dry plaster bonding layer to the tooling resin, inserting a reinforcement framework into the dam, and applying bulk casting plaster to complete the body of the mold.
According to the ""929 patent, the fabrication process forms a mold having a gel coat layer followed by two resin-glass fiber layers. A layer of an epoxy tooling medium mixed with dry plaster is then applied to the fiberglass layers as a binder coat, into which is immediately pressed a veiling of continuous strand glass fiber. A metal reinforcing framework is then placed in the epoxy-plaster layer before a bulk casting material comprising gypsum tooling cement is poured therearound to completely enclose the framework. A final layer of continuous strand glass fiber veiling is then applied to the wet plaster surface to prevent surface cracking during cure.
However, the mold disclosed by the ""929 patent may also experience problems in molding articles at high temperatures due to the use of materials with different coefficients of thermal expansion to form the mold. Further, the use of a cast-in reinforcing framework, which also serves as attachment points external to the mold, may cause distortion of the mold from misalignment. In addition, the application of the initial gel coat layer is applied only to areas of the mold where the subsequent glass fiber lamination layers are applicable. Catalyzed gel coat putty is then applied to the areas of the mold which the glass fiber lamination layers are not able to sufficiently cover. Thus, the surface finish of the mold disclosed in the ""929 patent may be compromised by having a discontinuous gel coat layer.
Thus, there exists a need for a method of making a cure tool wherein extensive handwork is not required to produce an acceptably smooth finish or surface quality on the cure tool, thereby resulting in a more determinable dimensional accuracy thereof. Further, there exists a need for a method of forming a cure tool wherein the tools or molds used in the process have good vacuum integrity. In addition, there exists a need for a method of forming a cure tool or bond jig wherein the tooling produced has good surface quality and vacuum integrity after being cured at room temperature. A method of fabricating a cure tool or bond jig would also be desirable wherein compensation for expansion of the facility tool at high temperatures is minimal or not required. However, if the facility tool were to expand as a function of temperature, it would be preferable for the mold to expand uniformly.
The above and other needs are met by the present invention which, in a preferred embodiment, provides a method of fabricating a cure tool for curing materials at elevated temperatures. The process forms a gel coat by applying layers of first and second resin surface coating mixtures to a tool surface of a facility tool which has preferably been previously coated with a release coating. As such, the process can begin by forming the facility tool having a tool surface defining a mold line corresponding to a pattern of the object to be molded. The tool surface is preferably formed of a tooling putty capable of a high quality finish to within the tolerances of the mold line. Once the facility tool is completed, a release coating is typically applied to the tool surface to facilitate separation of the cure tool from the facility tool. The release coating is then allowed to flash off for a predetermined period of time, such as at least 48 hours and, preferably, about four days.
According to one advantageous embodiment of the present invention, a mixture of a first resin surface coating is then prepared and de-aired to remove air trapped therein. A layer of the first resin surface coating mixture is then applied to the tool surface of the facility tool. The layer of the first mixture is then allowed to tack off for a predetermined period of time after being applied. Preferably, the layer of the first mixture is allowed to tack off for 30 minutes before being re-smoothed to promote substantially even application thereof to the tool surface. A mixture of a second resin surface coating is then prepared and then de-aired to remove air trapped therein. According to one particularly advantageous aspect of the present invention, the second mixture has a mix ratio by mass that is less than and, more preferably, is about half the mix ratio by mass of the first mixture. A layer of the second mixture is then applied to the layer of the first mixture after the layer of the first mixture has tacked off. Once applied, the layer of the second mixture is allowed to tack off for a predetermined period of time after being applied, preferably about one hour.
A rigidizing structure can then be applied to the layer of the second mixture to substantially fix both the layer of the first mixture and the layer of the second mixture in the form of the tool surface. The layer of the first mixture, the layer of the second mixture, and the rigidizing structure combine to form the cure tool. Preferably, the rigidizing structure comprises a plurality of plies of resin-impregnated carbon fiber applied to the layer of the second mixture and an eggcrate structure comprised of carbon epoxy members subsequently bonded thereto. Each ply of said plurality of plies is further vacuum-debulked after application thereof to promote wet out. Once the predetermined number of plies has been applied to the layer of the second mixture, a peel ply is applied. The layer of the first mixture, the layer of the second mixture, said plurality of plies, and said peel ply together comprise a facing sheet. Once the facing sheet is formed, it is bagged to the facility tool and preferably subjected to a decreased pressure of between about 25 and about 29 inches of mercury at room temperature for a period of about 48 hours to debulk and consolidate the facing sheet.
Once the facing sheet has been formed and cured, the peel ply is removed and an eggcrate structure comprised of carbon epoxy members is bonded to the facing sheet to complete the rigidizing structure and form the cure tool. Preferably, the coefficient of thermal expansion of the eggcrate structure corresponds to the coefficient of thermal expansion of the facing sheet. After the rigidizing structure has been applied to the facing sheet to form the cure tool, the cure tool is removed from the facility tool and post-cured; preferably free standing (without facility tool) and at a temperature of at least about 390xc2x0 F., to raise the glass transition temperature thereof. The resulting cure tool is useful for molding objects in high temperature autoclave environments, for example, at temperatures of about 355xc2x0 F. and pressures of about 100 psi. The process according to a preferred embodiment of the present invention produces better consolidation between layers of the materials forming the facing sheet, thereby eliminating leak paths therethrough and forming an improved mold tool by improving the vacuum integrity thereof.
Thus, embodiments of the present invention provide a cure tool or bond jig which does not require extensive hand work in order to attain a high quality finish with high vacuum integrity. Advantageously, facility tools and cure tools are produced which are sufficiently strong to withstand uneven forces applied thereto. Another advantageous aspect of the present invention is that, since the cure tool is comprised of carbon epoxy components, compensation for dimensional variation at high temperatures is not required when forming carbon epoxy molds from the cure tool since the coefficient of thermal expansion of the cure tool is the same as the mold. Further, the construction of the cure tool from carbon epoxy elements followed by the post-cure process advantageously results in a cure tool which will expand generally uniformly as a function of temperature. In addition, the peel ply offers excellent surface finish to allow bonding of the eggcrate to the facing sheet.